The present invention relates to a method of producing a dark heater for an electron tube having an indirectly heated cathode, particularly, a dark heater having a black appearance.
A dark heater has been used usually for efficiently heating a thermoelectron-emissive cathode in an electron tube having an indirectly heated cathode.
Such a heater comprises, in general, a core wire made of a high-melting point metal such as tungsten, a first coating layer made of an insulating material such as aluminum oxide (alumina) and covering the core wire and a second, external coating layer, i.e. a dark coating layer, made of a mixture of tungsten and aluminum oxide (alumina) particles.
For the formation of the dark coating layer, a dip coating method has been known. In this method, the specific gravity, viscosity, etc. of a suspension bath must be controlled strictly and, particularly, the first alumina layer must be dried completely so as to prevent the deterioration of insulation between the heater and the cathode due to penetration of tungsten particles contained in the suspension bath into the first alumina layer as disclosed in the specifications of U.S. Pat. Nos. 3,808,043 and 3,852,105.
However, an installment such as a furnace is necessary for drying the first alumina layer completely and, therefore, the production cost is increased. Further, in forming the dark coating layer by the dip coating method on the first alumina layer thus dried, a bath having a high viscosity must be used so as to prevent penetration of the tungsten particles contained in the bath into alumina according to a capillary phenomenon. However, a large amount of the bath liquid is applied to the first layer in such a case to form a thick dark coating layer. As a result, the heat capacity is increased, so that the rise time is prolonged and, particularly, in a color picture tube, imbalance in display among three colors is likely to occur and the coating layers begin to peel away and are scattered on the performances of the tube. Further, the bath liquid is apt to stay at a bend of a worked heater to cause dispersion of the emission characteristics.